Acoustic phase-shifting device



\ Jan. 13, 1931. w. P. MASON ACOUSTIC PHASE SHIFTING DEVICE I Filed Oct. 4, 1929 2 Sheets-Sheet 1 Jan. 13,

PHASE ANGLE l/V DFGPEES PHASE ANGLE //V DEGREES w. P. MASON ACOUSTIC PHASE SHIFTING DEVICE Fil d Oct. 4, 1929 2 Sheets-Sheet 2 FIG 4 20 so 40 .50 60 'IO 60 9o VALUE OF g- DEGREES FIG. 5

1o 4b 6 0 70 9b VALUE OF 2 4 //v 056/2555 2 ill BY%/VM A T TO/PNE Y Patented Jan. 13, 1931 uuirr.

WARREN 1?. Mason, or nnsronnlven'ivnw nRs YjAssmNoR' 'ro BELn'rELnPHoNn' LABORATORIES, INCORPORATED, or new YORK, N. Y.,QA oonronarioiv OF NEW YORK nooosrioriinsn-snirrnve nnvi'cnf'" Application filed October 4, 1929. Serial no. 397,201.

This invention relates to wave transmission,-and more particularly toacoustic devices for controlling the response character istics of speech transmission systems and the like.

An object of this invention is to correctphase, or transient distortion misslon systems.

inspeech'trans- Another object is to obtain economy-of space and material in the construction of phase distortion compensators. A

A problem of great importance arises in connection with the transmission of speech or other signals, over coil loaded, lines, wherein waves of difierentirequencies' are in general transmitted'with differenteiiecwhose overall phase characteristic is complementary to the phase characteristic ofth'e serve to connect it into the line, whereby the phase change in the combination is substantially proportional to the frequency. The use of an electricalnetwork, however, requires a large number of coils and condensers, and for the compensation of lines of any considerable length, the expense in- In accordance Withvolved is very great. this invention the electrical network is replaced by an acoustic device which has the property of being able to transmit waves of all frequencies without attenuation, but with different effective velocities; and which can J be proportioned so that the variable velocity characteristic substantially compensates that of a given transmission line. Telephone receivers or other suitable translating means, coupled to the ends of the acoustic device line it is desired to compensate. r

In the device of this invention, sound waves are transmitted through two paths'joined at each end either ina common sound channelor at the diaphragm of a telephone receiver.

Each of these paths includes an acoustic filter, a characteristic of which is that throughout the frequency spectrum there occur alternately, transmission and attenuatlon bands.

The filters are soproportioned that the transmission bands of'the one occur approximately the samefrequency rangesas the attenuation bands of the other. Wavs can thus pass freely through the one path or the other, or at the edges of the bands may be transmitted through both channels- By j properly proportioning the two paths the devicemay be made to pass Waves of all fre quencies without attenuation. In thisrespect the device acts like a simple uniform sound conduit, or any I other type of uniform .vvave transmission llne, but in the respect of the phasechange produced-in Waves ofdilferent frequencies-the. properties 'of'the device are quite different troin'those of a-uni'form line;

In the 'attenuatingband of a wave filter there is no change, with frequency, of the phase relation betweenlnput wave pressure and output waves, but throughout 'autransmitting' band, there is a continuous change in the phase relation, thischange being,in general,

non-uniformwith respect to frequency. This means that waves of different frequencies 7 traverse the Wave filter in different time in-' tervals or withdifierent efi-ectivefvelocities. In the devices of the invention the resultant frequencyrphase shift characteristicis that istics. together. 5 When the, filters are so proportioned that-the deviceis rigorously capableof passing wavesof alllfrequencies a smooth phase shift characteristic is obtained,

ofthe combined paths, and since the trans- Which canbesuited to the compensation of velocity distortion of loaded lines. The phasecharacteristic can also be adapted to simulate the phase characteristics of loaded lines and again, twoor'more structures, each an allpass phase shifting device in itself, can be joined in tandem to furnish a composite phase characteristic which is more suitable than the phase characteristic of any of the individual structures.

: a A feature of the invention is that the component wave filters are made up of elementsv eachof which is a short-section of-a uniform acoustic line, .or. has impedance properties similar to those of unlform acoustic hne, I In one embodiment the impedance of a short length of llneis simulated by an elast c membrane stretched across one of the sound 7 oaths such a membrane havin been'found to. have an impedanceof substantially the same character at all frequencies as that of a closed air column. Inanother embodiment, the impedance of a length of line, or air column, is simulated bya hollow circular" disc concentrically disposed in the soundp'ath. i

Fig. 1 illustrates a phase shifting device in I accordance with the invention in which the two sound paths are joined. together by telephone receivers; V

F ig.-2 is a detailed illustration of a tele phone receiver adapted to couple the sound paths inFig. 1;: w 3 v Fig. 3 shows. a, phase shifting device embodying the invention in which the two sound paths are coupled directly;

:Fig. 4: shows typical phase shift charac te r-istics obtainable in the devices of the in vention; v r r Y Fig. 5 illustrates1the phase characteristic of a loaded line and the compensating. characteristics of phase shifting devices accord- 7 ing to thisinvention; and. .1

Fig. 6 illustrates an acoustic phase shifter comprising a-plurality of sections. 1 1

Referringto Egg. 1, the phase shifting-section comprises two sound paths or. conduits] 10 and 1-1 of equal length joined at each endv by means of telephone receivers 12 andz 12. Each of the sound paths is provided at its midpoint with a side branch'wh-ich in path 10 'is an open ended air" column 13,and in path 11:.is a hollow concentric cylindrical chamber 14. The air column 13 is a length of pipe which is fastened at the end to a threaded flange 16 at the midpoint of conduit 10. Path 11 is made in two equal parts which are 'jointcd together in assembling. Each part 7 comprises alength of "conduit and half of -the enclosing walls of the associated cylin-' di ica l chamber. The two parts are joined by coupling together thetwo portions'of-the cylindricalchamberfwhich are} fastened by. screws 17 throughjfianges provided for the purpose around therini. The round conduits are connected tothe-receivers by fitting them into tight fitting bushings 18, 18" and 19, 19' provided in the outlets of the receivers. The component parts of the device should be proportioned in a manner to be hereinafter described. The device is adapted tobe connected between two portions of an electric line bymeans of impedance matching trans formers and 15. Fig. 2 is a detailed view in section of' a telephone receiver suitable for joining the ends of the sound paths 10 and 11 in Fig. 1. This receiver is of the moving coil type, the driving. force being appliedto a diaphragm V 20by a pair of helical coils 21 and. 21 rigidly ing for the movement of the fastened thereto and located in air gaps between the poles of electromagnets 22 and 22'.

Thesetwo electromagnetics are clamped towindings 26 and26" which are to be connected-to a source of direct current such as a storage battery. The receiver is provided with two sound outlets 27 and 27 one on each side of the diaphragm and, since the conduits which are coupled together bythe receiver are in general of different sizes,: the

diainetersof the sound outlets aremade'to correspond. The plunger portion of the diaphragm s made convex outward on both-sides so that it is adapted to propagate waves,

equally well in each direction. 1 This construction of the diaphragm is most easily ob.-

tained by makingthe diaphragm stamping V termediate annular sections of the diaphragm, thejsound passages through the centers of the magnets being made divergentand provided for, this purposewith conical cen- Fig. 3 .-i llustrates a modification ofthe phase shiftin section of Fig. 1. It comprises two sound paths '11 and 30, the ends of which -meet directly in common channels 32 and 32. "Path '11. is similar to-path ll in Fig.

convex on one side and then cementingan. faumharyconvex plunger portion 29 o'n the other s de;- .Thesound is taken ofi'fromi-ins' tml P g 28 and 28 attached'to the magnets- .by webs.

1. Path 30, however,- instead of having at its midpoint a side branch tube, has an elastic membrane 33 stretched across the channel.

To enable the membrane to be put in place and stretched, path 30 is constructed in two parts which are joined at the membrane.

Flanges 3a and 35 are provided at the ends of the component parts of the sound path, the,

flange 84 sliding over flange 35. The edge.

of the membrane is placed between the two flanges, which are fastened together by machine screws 36 so that the tension of the membrane canbe regulated by tightening or loosening the machine screws. This embodiment requires no acousticalelectrical link to join thentw'o paths. but if it is desired to incorporate' thQSQQUOIl lIlE-O an electrical line,

telephone receivers must be used in thecom- 111011 channels 32 and 82. As willbe seen,

it is usuallywlesirable to use more than one section of the structure of Fig.1 or Fig. 3' in which case the telephone receivers need be usedonly in connection with the two end quantity,

later." j i It can be shown that the propagation constants of the sections of'both Fig.1 and Fig.

sections, in a manner which will'he shown 3 are equal when certain conditions which will later be disclosed, are fulfilled, and are expressed by the equation:

ZwL i wL wL cos icot T tan where P denotes the propagation constant The acoustic impedance as usedherein is defined as the ratio of the excess pressure intensity to the volumetric flow of air per second over the cross section of the path at the point under consideration; the acoustic characteristic impedance of any guniform conduit is the impedance which woulctbemeasured at the input end of an 'infinite length of suchaconduit and is equaltothe POW).

divided by the cross sectional area of the con duit. Thus, p

. Z1, s." j

in which P denotes the'atmospheric pres- 7 sure 'y dencites the ratio of the specific heats of'air'; p denotesthe densityfof air; and 7 S and Si denote the cross sectional areas of the respective soundconduits corresponding to Z and Z I qu n y, Pwp

is'a constant about equ'alfto v 43 when air-is the medium,

The structures of Figs. 1 and-3 are ver-v I similarly arranged, Fig. 1 showing two sound paths joined in series by telephone'receivers, and Fig. 2 showing two sound paths in paral- In accordance with this invention the i npedances connected at the midpolnts of the sound paths are so miter-related that the 1m-' 7 pedances as from the terminals of the structures-are the same as though no imped- Thus, the characteristic impedance, denoted Z of the section of-Fig. 1 is expressed by Z =Z +Z =M (4 p V 1 2 while the characteristic impedance, denoted Z, of the structure in Fig. 3 is given by The conditions which must be fulfilled order that'Equations (1), (4:) and (5)"applyf are, first, that the impedance located at the midpoint of each soundconduit shall be equal to that of an aircolumn havinga length L; and second, thatthe iinp'edances, of the air columns or their equivalents must be related to the cross section of the upper and lower ances were connected to the main soundpathsa sound paths in a manner to be explained presently. n

F or-the structure ofFig. 1, it has'be'en found that the midpoint impedances must consist of an open ended air column in shunt with one path and a closed ended air column in shunt with the other, or else the equivalents ofthese air columns. For the structure of Fig. 2, it has been found that the midpoint impedance of one path must be a closed ended air column in-serics, and of the other, a. closed ended air columniin' shunt, or their equivalents.v

Each sound conduit together with its asso ciated midpoint impedance constitutes an nite'number of alternate transmitting and alternating regions throughout the frequency seale *In a transmittingregion there is a change in the phase relationship between the inputand the output waves, while in an attenuating region there is no phase change. In the structure of Fig. 1, the path including the openair coluinn in shunt is a high-pass filterhecause itattenuates waves in the region around zero frequency'and freely transmits waves of higher frequency. Gr. the other hand the path including the closed air column in shunt is a low pass filter because it freely transmits waves around zero. frequency. Considering Fig. 3, the path containing the membrane isa high pass filter while the other pathis a low pass filter. In order that the 'acou'stic wave filter characterized by an infisection asa whole shall freely pass waves of? all frequencies, and in consequence have a uniform phase sh ft characteristic with fire quency, it is essential that. the transmitting regions of the two component filters overlap each other. This is the condition'wh ch exists.

, in which Z3 denotes thejcharacteristic impedance of thea-ir column, or its equivalent,

connected to the midpoint of the path having the impedance ZL, i. .e. the upper path in V 1 v V thengure; and

Z denotes the characteristic impedance of the air column, or the equivalent, connected to the lower path, having the im'pedance In the case of the section in Fig. 3, the mid point'impedances of the twosound conduits are given by where Z and Z denote the characteristic impedances of the air columns, ortheir equivalents, connected to the nndpmnts of paths and 11 respectively.

\Vhile it has been shown that the impedance connected at the midpoint'of each sound conduit should be equal to that of an air column, nevertheless it is not always possible or practicable to employ an air column. "Under such conditions an equivalent impedance which can be practically constructed, must be utilized. For instance, it is not possible to connect a closed :ended air column in series with a sound conduit; nor is it practical to awe heare rena-ac:

connect as ajside branch a column having a larger diameter than the diameter ofgthe sound conduit. So to overcome these difficulties, equivalent impedance devices'are employed. An elastic membrane stretched across the conduit has been found to be equivalent to aclosed ended sound'tube in series with the conduit, the equivalence being close for frequencies up to the'first resonance of the membrane. Such an elastic membrane is what is used to furnish the impedance at the midpoint of path 30 in Fig. 3. The impedance of a stretched circular membrane of this type is expressed 'by'the' Bessel equation where Z denotes theimpedance-of the membraneyi -r: '2 a .1 J

J.. and J are Bessel functions of the first kind; ,1

p denotes the surface :density, i. e. the weight of one square centimeter of "the' membrane; T'denotes the tension per inch of length V R denotes the radius of themembrane; --'lhe impedance of the'membrane can be made very nearly equal to-tha-t of the corresponding closed end tube in the range extending from a very lowfrequency up to the first resonant frequency by making the imped-.

ances of the membrane and of the tube equal atlowlfrequency and by making the resonant frequencies equal. j d

Likewise, the hollow cylindrical chamber 14in pathll is used in lieu of a closed ended side branch tube'when the tube would be too large. 'The impedance, denoted Z5, ot-the chamber is expressed the Bessel equation in which R denotes the radius of the disc;

denotes the radius of the conduit .to which the disc is attached; i denotes "the width'of the disc in the axial direction; Y and YiareBe ssel'functions of the second kind; jfisthe imaginary operator. In the .same manner that the impedance of themembrane is made equivalent to that of a closed ended tube, the impedance of the cylindrical chamber is also made equivalent to a closed ended tube, namely, by constructing the chamber so that it has the same resonance point and its impedance is equal to that of the corresponding tube at very low frequency. Referring to the propagation constant P given in Equation (1), it can in general bev expressed by P=A+jB in which 1-3;, the real component expresses the attenuation of the section; f i B, the imaginary component denotes the phase shift and is equal to per section An inspection of Equation (1) shows that the attenuation component is zero and that the propagation constant consists only of a phase shift, that is to say, the devices freely pass waves of all frequencies, but produce a change in the phase relation between the waves at the input and at the output ends of i the section. This phase relationship changes with frequency and the actual phase characteristic is dependent upon the ratio The ordinates denote the degrees of phase shift of a single section and the abscissae are measured in terms of Which is the phase angle of a single of" :0 i

the structure extending from one end to the midpoint. The characteristics for which the ratio ranges from 1 to 3 are the type suitablefor compensating for the phase distortion of loaded lines, While the characteristics for which ranges from Ito- are

are generally. suitable for simulating the .phase characteristics of such lines The following example is given toshow method of designingua section of a phase shifter of the type of this invention. Suppose it is desired to obtain the phase characteristic for which i n. V

and for which the phase shift is 540" at 3000 c. p. s, by means of the type of structure shown in Fig. 3. Suppose, further,'that it is desired to make the common channels 32 and 32 1.7 8 centimeters in diameter. This diameter will ordinarily be chosen to fit an existing telephone receiver. The length L is determined from'the fact that when the phase shift of the entire section is 540. Since the frequency is 8000 c. p. s the length L is found to be 2.88 cm. Since Z ZI1 and s1+s2==.s, it. follows that I s s 7 i as i=3; S ;and S -Hm From. these relations Si 180 sq. cm. and- S =.62 sq. cm. Since thecharacteristic impedance of a uniform conduit is i/P rp divided by'the area of the cross section, the characteristic impedances oft-he two paths 43" Z 28.1 ohms and g L,=%%? 69-4 5 respectively. From Equation (9) is found to be 2 88 ohms; 7 If a closed ended tube were used to furnish its area S. would be p f p v '43 I i a l4.9 sq. cm.

Obviously this tube would be-too large to connect to a pipe whose cross section is only .62 sq; cm,., so the equivalent cylindrical chamber will be used instead, The chamber will be made" 'equivalentto the tube by making. it resonant at the same'frequency and by male V os ing the low frequency impedance 'of the chamberand tube alike. It will then be assumed that the impedance of the chamber is equal to that of the tube up to the first resonance.

The cylindrical cha 'beriis resonant-when 1ts impedance, given by- Equation (11), is

equal to zero. Letting Z,;=0, the'solution of Equation (11) is Now the closed'ended tube, the length of which is L, is resonant when 2L1 q 7 c 2 From these last two relations ni L I or R =239 cm. v

At low frequency, Equation (11) becomes Z im rs Letting this equal the low frequency impedance of the closed ended tube which is and solving for the axial length ofthe cylindrical chamber J The series characteristic impedance I "5L Z IBOt-F' The membrane impedance as'givenby Equation (10) becomes zero when while the closed tube impedance Zero when The solution of the last two equations gives the membrane-.jradius,

Rm: 2L(2.4o4 s) At very low frequency, the impedance of the equivalent closed ended tube is i L3 wL while the impedance of the membrane is and" i V e V (2.4048) R LZ' 21m Substituting 20.57. for Z p is found to be equal to .001575 Rm? It is best to choose a minimum thickness for the diaphragm and then solve for the radius, thisbeing satisfactory as long"- as the radius works out to v be equal to or greaterjthan the radius of the tube across which it is stretched, The membrane thickness will be made one-halfmil or .00127 cm., which is a thickness obtainable in a foil of aluminum. The specific density of aluminum is 2.75. The thickness is equal to surface density divided by the specific density, and from this relation the radius of the membrane is found to be 1.49 c1n., which is somewhat larger than the radius of the conduit across whichit is stretched.

For accurate simulation or compensation,

' it will oftenbe found desirable to employ the composite phase characteristic of phase shifting sections having different individual phase characteristics, for by so doing, a greater control is obtainable over the composite, characteristic than if all thesections are uniform. Fig. 5 illustrates phasecharacteristics which show how the phase distortion of six sections of loaded cable can be compensated by two sections of the acoustic phase shifters of this invention. Curve a represents the 1 phase characteristic of the six sections of loaded cable, a section ofcable being a loading coil and its associated spanof cable. The cut-off frequency of the cable is about 3000 c. p. s. and this frequency corresponds to the value of on the axis of abscissae which is equal to 90. The frequency is proportional to so the other frequencies as measured along the abscissae bear the same relation to the value of that 3000 c. p. s. bears to an of 90. It is well known, that a trans-Q mission line to be free from phase, or velocity,

distortion, must have an over-all phase shift characteristic which increases, uniformly with frequency, that is, the characteristic must be.

a straight line. It is found that two sections of the acoustic phase shifting devices of this invention, one section having the ratio the other having when connected in tandem with the six sections of loaded cable, will cause the over-all phase characteristic to be nearly a straight line. Curve 7) and 0 show respectively the characteristics of the acoustic sections with and Since the sections are in tandem the phase angles are additive; the sum of the phase angles of the two acoustic sections is curve d. Curve 0 is obtained by adding curves (1 and cl and it represents the over-all phase shift. It is noted that curve e is nearly a straight line, so the system is practically free from phase distortion.

Loaded lines are generally of great length and so have many loading sections, inasmuch as the loading co1ls are usually spaced at intervals varying from one to two miles. It

will usually be more convenient to situate the phase correcting devices at widely separated. points than at intervals of only a few loading sections. When the phase correctors are very widely separated the number of phase shifting sections connected togetherwill have to be much greater than if they were situated at every few miles. Fig. 6 shows a preferred arrangement of a plurality of phase shifting sections joined in tandem, this arrangement same ratio being suitable for. loading a line at long intervals. The sections are arranged-in two groups designated and 61 which are'joined in tandem by a four-way pipe coupling 62.

Group 60 comprises a plurality of sections of thetype' shown in Fig. 3 in which the ratio is equal to 3, while group 61 comprises sections of the same type inwhich this ratio is 'sive membranes 65 and between successive side branches 66 are equal to 2L, since half of the length, that is,'L, is associated with each membrane or side branch. The length of conduit from'each end membrane or side branch to the common coupling is equal to L, as in Fig. 3. .In this'manner any'nu'mber of sections of different proportions may be'coupled,since any'numb'er of groups may be used and any number of sections may constitute a group. Sections of the type of Fig. 1 couldbe combined in the same manner, but instead of acoustic couplings betweengroups, telephone receivers would i be necessary. Moreover, groups of the type of Fig. 1- could be combined with groups of'the type-of Fig.

Regardless of which type of phase shifter is used it'is always necessary to use telephone receivers to connect the ends of the main conduit into an electrical line. c WVhatis'claimedis:

f1. An acoustic phase shifting system comprising two sound channels, meansfor im' ,presslng sound waves simultaneously on one end of each channel, and means for'receiving sound waves from the opposite ends of each sound channel simultaneously, each of said channels including branches, whereby said system has'a non-linear phase shift characteristic. V i I 2. An acoustic phase shifting section comprising two paths, each path including afilter having a branch in communication with the path whereby each path is characterized by an infinite number of alternate transmi sion and attenuation bandsthroughout the frequency spectrum, said paths being joined together at each "end, one end being adapted to receiv e wave energy, the other to'deliver wave energy. 1

3. Anacoustic delay systemaccording to claim 2 in which one of said filters is a high pass filter, the other a low pass filter.

' 4; An acoustic relay system comprising two acoustical-electrical transducers and two sound paths communicating between said transducers, each path including a filter sof related to each other that the transmitting regions of one overlap the attenuating regions of the other whereby the system is substantially an all-pass system over a wide range of frequency. g

5. An acoustic phase shifting device comprising two channels joined at their ends each channel having at its midpoint an impedance equal to that of an air column whose length is equal to the distance along a channel from one end to the midpoint.

6. An acoustic phase shifting device comprising two parallel channels joined at each end, one channel including effectively an open circuited line in shunt, the other channel including effectively an open circuited line in series.

7. A acoustic phase shifting device according to claim 6 in which said shunt line is equivalent to a closed ended tube branching from the channel.

8. An acoustic phase shifting device according to claim 6 in which said series'line is provided by a membrane stretched across the channel.

9. An acoustic phase shifting section comprising two parallel sound channels of equal length, there being a hollow disc situated at the midpoint of one channel, the impedance of said disc being equal to that of a closed ended tube having a length equal, to half the length of the channel, and a stretched membrane across the other channel at'its midpoint, which is equivalent to a closed ended tube of half the length of the channel effectively in series, whereby said section is an all-pass structure having a continuous phase shift characteristlc.

10. An acoustic phase shiftlng section according to claim 9 in which the ratio of the cross section of the two sound channels is fixed by preassigned phase shift characteristic which the device is to have.

11. An acoustic phase shifting system comirisinga plurality ofsections according to claim 2. V

12. An acoustic phase shifting system comprising a plurality of sections according to claim 2, the ratio of the characteristic 1mpedances of the branches of one section losing different from the same ratio'oi' another section.

In witness whereof, I hereunto subscribe my name this 1st day of October, 1929.

WARREN P.- Mason. 

